Electrical submersible pumps (“ESP”) are used to pump wellbore fluids from the depths of the earth to the surface. A typical ESP has a motor, a seal section, and a pump. The motor rotates a shaft inside the seal section. The seal section shaft is connected to the pump. The ESP pump is typically an impeller pump having multiple stages. Each pump stage has an impeller and a diffuser through which wellbore fluid travel. In operation, wellbore fluids enter the first impeller and are accelerated by centrifugal force out of the impeller into the adjacent diffuser. The diffuser then reduces the velocity of the wellbore fluid, converts the high velocity to pressure, and directs the fluid into the next impeller. The pressure of the wellbore fluid is increased with each successive stage as described above, until the fluid is discharged from the pump into tubing that carries the fluid to the surface.
A central pump shaft is connected to the seal section shaft. As the motor rotates, it ultimately causes the central pump shaft to rotate. The central pump shaft passes through each impeller. Keys or splines on the shaft engage corresponding slots on each impeller so that the impellers rotate with the shaft. Spacers are frequently required between the impellers so that the impellers are properly spaced to engage the diffusers.
An electrical submersible progressive cavity pump (“ESPCP”) having a single stator and a rotor may also be used. A typical ESPCP has a motor, a seal section, and a pump. An optional gearbox may also be included. A PCP is a positive displacement pump in which the rotor and the stator have cavities that are filled with fluid. As the rotor is rotated by the motor, fluid is moved upward. For discussion purposes only, ESP is used throughout with the understanding that either an ESP or ESPCP can be used.
Multiple ESP pumps may be connected in series and used in a single well. The ESP pumps are typically driven by a single motor with the shaft running through each of the ESP's. During operation, multiple ESP pumps, or tandem pumps, arranged in this manner provide additional lift that may be necessary to lift the wellbore fluids to the surface.
In wells where tandem pumps are deployed, there may be times during the life of a well where a reduced number of stages or a single ESP pump may be required to lift the fluids. Running the additional ESP pump or increased number of stages is inefficient and expensive. However, to disengage the ESP pumps from the shaft, the ESP string typically requires the ESP system to be pulled out of the well. This is an expensive proposition because production must be stopped during this procedure and subsea replacement can cost millions of dollars.
It would be advantageous to selectively engage or disengage an ESP pump from a drive shaft without pulling the ESP assembly from the well.